Multiparty communication complexity

In the 2–party communication game, introduced in in 1979, two players, P₁ and P₂ attempt to compute a Boolean function

$f(x_{1},x_{2}):\{0,1\}^{n}\to \{0,1\},\ x_{1},x_{2}\in \{0,1\}^{n'},\ 2n'=n$

Player P₁ knows the value of x₂, P₂ knows the value of x₁, but P_i does not know the value of x_i, for i=1,2.

In other words, the players know the other's variables, but not their own. The minimum number of bits that must be communicated by the players to compute f is the communication complexity of f, denoted by κ(f).

The multiparty communication game, defined in 1983 , it is a powerful generalization of the 2–party case: Here the players know all the others' input, except their own. Because of this property, sometimes this model is called "numbers on the foreheadˇ model, since if the players were seated around a roundtable, each wearing their own input on the forehead, then every player would see all the others' input, except their own.

The formal definition is as follows: k players: P₁,P₂,...,P_k intend to compute a Boolean function

$f(x_{1},x_{2},...,x_{n}):\{0,1\}^{n}\to \{0,1\}$

On set S={x₁,x₂,...,x_n} of variables there is a fixed partition A of k classes A₁,A₂,...,A_k, and player P₁ knows every variable, except those in A_i, for i=1,2,...,k. The players have unlimited computational power, and they communicate with the help of a blackboard, viewed by all players.

The aim is to compute f(x₁,x₂,...,x_n), such that at the end of the computation, every player knows this value. The cost of the computation is the number of bits written onto the blackboard for the given input x=(x₁,x₂,...,x_n) and partition A=(A₁,A₂,...,A_k). The cost of a multiparty protocol is the maximum number of bits communicated for any x from the set {0,1}ⁿ and the given partition A. The k-party communication complexity, C^(k)_A(f), of a function f, with respect to partition A, is the minimum of costs of those k-party protocols which compute f. The k-party symmetric communication complexity of f is defined as

$C^{(k)}(f)=\max _{A}C_{A}^{(k)}(f)$

where the maximum is taken over all k-partitions of set x=(x₁,x₂,...,x_n).

Upper and lower bounds

For a general upper bound both for two and more players, let us suppose that A₁ is one of the smallest classes of of the partition A₁,A₂,...,A_k. Then P₁ can compute any Boolean function of S with |A₁|+1 bits of communication: P₂ writes down the |A₁| bits of A₁ on the blackboard, P₁ reads it, and computes and announces the value f(x). So, we can write:

$C^{(k)}(f)\leq {\bigg \lfloor }{n \over k}{\bigg \rfloor }+1.$

The Generalized Inner Product function (GIP) is defined as follows: Let y₁,y₂,...,y_k be n-bit vectors, and let Y be the n times k matrix, with k columns as the y₁,y₂,...,y_k vectors. Then GIP(y₁,y₂,...,y_k) is the number of the all-1 rows of matrix Y, taken modulo 2. In other words, if the vectors y₁,y₂,...,y_k correspond to the characteristic vectors of k subsets of an n element base-set, then GIP corresponds to the parity of the intersection of these k subsets.

It was shown that

$C^{(k)}(GIP)\geq c{n \over 4^{k}},$

with a constant c>0.

An upper bound to the multiparty communication complexity of GIP shows that

$C^{(k)}(GIP)\leq c{n \over 2^{k}},$

with a constant c>0.

For a general function f, one can bound the multiparty communication complexity of f by using its L₁ norm as follows

$O{\Bigg (}k^{2}\log(nL_{1}(f)){\Bigg \lceil }{nL_{1}^{2}(f) \over 2^{k}}{\Bigg \rceil }{\Bigg )}$

Multiparty communication complexity and pseudorandom generators

A construction of a pseudorandom number generator was based on the BNS lower bound for the GIP function.